The active ingredient in a drug that's expected to become a popular treatment for multiple sclerosis has been linked to four European cases of a rare but sometimes fatal brain disease called progressive multifocal leukoencephalopathy (PML). The ingredient, dimethyl fumarate, is used in a drug called Fumaderm that was approved in Germany in 1994 to treat the skin condition psoriasis. It is also in a different but closely related medication called Tecfidera, which was just approved by the U.S. Food and Drug Administration in March for the treatment of multiple sclerosis (MS). It is known as a fumaric acid ester, which is commonly used as a food additive and has been used to treat psoriasis in Germany for 30 years.